1. Field of the Invention
The present invention pertains to a method of reducing contaminants in a semiconductor processing environment. In particular, the present invention pertains to a method of preventing particulates of iridium (Ir) and other noble metals, such as platinum (Pt), from adversely affecting an iridium/iridium compound etch process performed in a plasma etch chamber.
2. Brief Description of the Background Art
Ferroelectric random access memory (FeRAM) cells have been introduced as a future generation of very high density memory cells, potentially at the giga bit level and beyond. Storage capacitors in such FeRAM cells require new materials for their electrodes and dielectrics in order to meet increasingly small design requirements. Lead zirconium titanate (PZT) is a leading candidate for a dielectric material for FeRAM cells. PZT, referred to as a high dielectric constant (k greater than 20) material, has been found to have excellent characteristics for use in very high density storage capacitors. When storage capacitors are formed with a PZT layer sandwiched between electrodes made of metals such as aluminum and aluminum alloys, a longer data retention time is achieved than with conventional storage capacitors. However, the retention time gradually decreases, requiring frequent data refresh operations to be performed in order to safely retain data within the storage capacitors. Therefore, conventionally used electrode metals have proven to be unacceptable for use with PZT in the fabrication of storage capacitors for use in future generation high density memory cells.
There are two basic requirements for storage capacitors for use in very high density memory cells: 1) longer retention time; and 2) tolerance to a large number of data refresh operations without significant deterioration of the charge characteristics during the lifetime of the memory cells. For example, for non-volatile memory (NVM) applications, the desired data retention time is over 10 years; for DRAM applications, data refresh operations may be performed more than one million times over the lifetime of the storage capacitors.
Recently, iridium and iridium compounds have been evaluated as new materials for electrodes of storage capacitors. Iridium and iridium compounds are known to have several advantages over conventional metals such as aluminum, including: 1) use of conventional chemical vapor deposition (CVD) methods to form the electrodes; 2) ability to be plasma etched; 3) good adhesion characteristics to PZT (i.e., chemically and physically stable interfaces); 4) good bonding characteristics with other metals for interconnections (i.e., good electrical contacts); and 5) stable and reliable operations at high temperatures when working with other elements and devices.
Storage capacitors formed with iridium and iridium compounds as electrodes and PZT as a dielectric material show excellent characteristics in terms of data retention time and allowable refresh operations. As a result, storage capacitors formed with PZT, iridium, and iridium compounds are a viable candidate for the future generation of storage capacitors.
One of the problems encountered with forming such a storage capacitor is that a significant number of contaminant particulates are generated during the etching process and remain inside the plasma etch chamber. Iridium particulates make up a large portion of the particulates observed in a plasma etch chamber subsequent to the formation of storage capacitors of the kind described above. FIG. 5 shows the composition of particulates, measured by energy dispersion spectroscopy (EDS), on a wafer that has been processed in a plasma etch chamber. Iridium particulates, in particular, remain even after a purge operation, and can seriously affect subsequent wafer processing operations.
The mean time between chamber cleaning operations is measured by Mean Wafers Between Cleans, MWBC. An economically feasible MWBC is about 400 to 500 wafers between cleaning operations, with the industry goal for mass production as high as 1000 wafers between cleaning operations, assuming a single wafer load per etch process per chamber. The use of iridium and iridium compounds in the formation of electrodes results in a significant reduction in the MWBC, sometimes to as low as 10 wafers, which makes forming electrodes using iridium and iridium compounds economically impractical.
U.S. Pat. No. 6,020,035, to Gupta et al., discloses a method of depositing a seasoning layer on surfaces of a substrate processing chamber, to cover contaminants (primarily fluorine-containing) which may be absorbed within the walls of insulation areas of the chamber and to block the release of these contaminants from chamber walls. Unfortunately, this conventional seasoning method was found to be ineffective at reducing the amount of free iridium and iridium compound particulates found floating within a plasma processing chamber after seasoning, even after cleaning with a purge gas.
Therefore, there is a need for a method of controlling undesirable residual iridium and iridium compound particulates and platinum particulates remaining within a plasma processing chamber, even after cleaning using methods currently known in the art.
The present invention provides a method of preventing particulates of iridium, iridium compounds, platinum, and other noble metals, which are generated during a metal etch process, from adversely affecting a subsequent metal etch process performed within the same plasma etch chamber. The method comprises exposing interior surfaces of the plasma etch chamber to a seasoning plasma generated from a gas mixture comprising at least two gases selected from the group consisting of BCl3, HBr, and CF4, for a time period sufficient that a subsequent measurement of particulate count on a monitor silicon wafer indicates an acceptable particulate count.
Reduction in iridium particulates in the plasma etch chamber after performing the seasoning method of the invention is significant, allowing as many as 400 to 500 wafers to be processed between cleaning operations. Such a significant increase in MWBC results in reduced processing costs, as well as improved yields.
Also disclosed herein is a method of forming a storage capacitor in a plasma etch chamber, comprising the following steps: a) exposing interior surfaces of the plasma etch chamber to a seasoning plasma generated from a gas mixture comprising at least two gases selected from the group consisting of BCl3, HBr, and CF4; b) purging the plasma etch chamber of remaining seasoning gas mixture; c) loading a substrate having at least one layer of a noble metal such as iridium or platinum formed thereon into the plasma etch chamber; and d) plasma etching the at least one layer of the noble metal.
Also disclosed herein is a method of forming a storage capacitor in a plasma etch chamber, comprising the following steps: a) loading a substrate having at least one layer of a noble metal such as iridium or platinum formed thereon into the plasma etch chamber; b) plasma etching the at least one layer of a noble metal; c) removing the substrate from the plasma etch chamber; d) cleaning the plasma etch chamber using a purge gas; and e) exposing interior surfaces of the plasma etch chamber to a seasoning plasma generated from a gas mixture comprising at least two gases selected from the group consisting of BCl3, HBr, and CF4.
Also disclosed herein is a method of preventing platinum particulates generated during etching of a layer of platinum in a plasma etch chamber from adversely affecting an etch process subsequently performed in the plasma etch chamber. This method comprises exposing interior surfaces of the plasma etch chamber to a seasoning plasma generated from a gas mixture comprising at least two gases selected from the group consisting of BCl3, HBr, and CF4.